AEROGRAPH, OR AIR BRUSH (Fr., Aerographe ; Ger., Aerograph) A mechanical sprayer working by means of compressed air, and used for finishing and work ing up both prints and negatives ; invented by Charles L. Burdick, of Chicago, in 1892, and introduced into England a year later. It is capable of great technical possibilities, and pro duces effects varying from peculiarly soft and beautiful graduations to strong and vigorous work. At will, and in successive instants of time, the operator can draw lines or wide bands of colour, and shadows, either soft and delicate, or hard and coarse. The complete outfit includes the handpiece, or fountain air brush, an air pump with compressed air reservoir, an air pressure gauge, rubber tubing, liquid colour, etc. ; inas much as a high and uniform air pressure is essential to the best results, n motor-driven air pump is superior to the foot-operated one, and both kinds are manufactured. Cylinders of com pressed carbon dioxide (" carbonic acid gas ") may be used in place of the pump with equal convenience. When using the foot pump the pressure obtained is about 15 lb. per square inch, and when using the motor pump from 30 lb. to 40 lb. per square inch, this producing a much finer grain.
The illustration shows the appearance of the aerograph. Air is pumped into a chamber connecting with the handpiece by means of rubber tube at A. Finger pressure on a button B opens a valve and admits the air, which sucks the liquid colour from the reservoir C and throws it from D in the form of a fine spray, over which the operator has complete control. The spray is regulated and stopped by a needle-like rod E worked by B.
In the management of the aerograph, scrupu lous cleanliness is always necessary. Keep the pencil in the case when not in use, and before fitting it up for service, pump air into the cylinder, squeeze the tube for a moment or so, and then release so as to allow the dust inside the tube to blow out. Use the colour thinly, and go over the work several times to get a fine, even grain. Use fresh colour for every occa sion, and change the water frequently to avoid dust, which otherwise will cause the colour to splutter. Before and after using the instrument, pass two or three lots of clean water through it, and clear away any accumulation of paint with a wet brush. To adjust the needle, just fit it easily to the platinum point where the valve lever is as far forward as it will go. Do not jam it hard and push far in, otherwise too much colour will be ejected on pressing down the valve. Also see that the rubber tubes are free from kinks or bands. Having mixed the colour in a clean saucer, transfer to the reservoir by means of a brush. Spray a little in the air before treating the original, to make sure that all the cleaning water is expelled and the brush is working properly. Hold the aerograph about
6 in. distant from the original, press the lever down and slightly backwards, move it horizon tally with gliding sweep from left to right, beginning at the top left-hand corner, and releasing the valve at the end of each journey until it has travelled in this way downwards over the whole space to be covered. The air brush unaided does not produce sharply defined lines and edges ; paper masks must therefore be used to obtain these, or if the background has merely to be painted out, then the parts that are to be protected can be covered over with a special preparation, " Masklene," supplied by the makers of the aerograph ; the colour is sprayed on, and the protected parts are then cleaned with n pledget of cotton wool soaked in benzine.
In process work, the aerograph is extensively employed for working up originals for repro duction, especially photographs of objects for catalogue illustration. Backgrounds are in most cases put in with the aerograph and vignetted off. Sometimes the main object is cut out of the photographic print with scissors or a sharp knife, then mounted on cardboard, and a back ground and other detail put in. Another method largely adopted is to stop out portions on which the aerograph spray is not to be applied, the stopping-out medium being of such a nature that it can afterwards be removed with out injuring the rest of the drawing. The medium used may be either of a greasy nature, such as vaseline, which may be afterwards cleared away with benzol, or it may be a cellu loid varnish, which may be removed with amyl acetate or other solvent of celluloid. India rubber solution and yolk of egg are other sub stances used for the same purpose. The former can be peeled off by rubbing with the ball of the finger, whilst the latter will flake off. In either case the colour applied by the aerograph conies away with the medium, and leaves quite clean the portions which have been covered. Larger surfaces may be stopped out by cutting out masks of tracing paper and attaching these temporarily to the print with rubber solution. The colours used should be mixed to match as nearly as possible the tints of the original to be worked up. Chinese white should be avoided, as it photographs darker than the white papers on which it is applied. Albanine, Ullmanine, and Blanc d'Argent are good whites to use for this work. Lampblack and " process black " are the blacks commonly employed. For large lithographic work, such as posters, a larger hand piece is employed, which will give a coarser spray and will not clog with the transfer ink necessi tated by the lithographic process.